The present invention relates to a free-piston engine having a fluid pressure unit according to the preamble of claim 1.
In a known free-piston engine having a hydraulic unit (see U.S. Pat. No. 3,606,591), a second chamber portion of a second chamber is connected to the compression pressure accumulator through a connecting channel having a two-way valve. A compression stroke starts if a two-way switch is switched from the closed position to an open position. During the first part of the compression stroke hydraulic liquid flows through said two-way valve until, in a second part of the compression stroke, a connecting channel of a first chamber portion of the second chamber takes up the main part of the liquid flow from the compression pressure accumulator. In this prior art piston engine, very high and also conflicting demands are made upon the two-way valve. Then the two-way valve should have a very short switching time, ca. 1 ms, on the one hand requiring a small valve, and the two-way valve should have a relatively large flow capacity on the other hand in order to restrict the loss during the first part of the compression stroke. It is hardly possible to comply with these high and conflicting requirements so that in the present free-piston engines the efficiency is adversely affected by the loss of energy in the two-way valve, while the power of the engine is restricted by the slowness of the two-way valve.
It is an object of the invention to provide a free-piston engine having a fluid pressure unit in which said problem is solved in an effective way.